Electrospray ionization (ESI) sources are used to produce gas phase analyte ions for analysis by analytical instruments, such as mass spectrometers. Under common electrospray ionization mass spectrometry (ESI-MS) conditions most analytes are not directly affected by the electrochemical process occurring while passing through the ESI source. Nonetheless, electrochemical reactions of analytes of interest can and do take place. These electrochemical reactions can alter the analyte molecules such that the ions observed in the gas phase have a different mass, charge, or both, from the original analyte molecule. Planned analyte electrolysis can be very advantageous, providing the ability to create novel gas-phase ionic species, probe analyte redox chemistry, and perform electrochemical ionization.
In general, problems with ESI source analyte electrolysis arise where the analyte has a low oxidation potential or high reduction potential relative to the surface potential generated at the electrode surface in order to produce the current required for ionization. As used herein, the phrase “low oxidation potential or high reduction potential” is used to refer to the problem of electrolysis of low oxidation potential analytes in positive ion mode ESI and the problem of electrolysis of high reduction potential analytes in negative ion mode ESI. Several reports propose to eliminate this effect using homogeneous redox buffer solutions or sacrificial electrode materials to buffer the potential of the emitter to a degree where analyte electrolysis does not take place. Unfortunately, both methods introduce products of the buffering reaction to the solution that may have unwanted effects. For example, the hydroquinone oxidation product benzoquinone can react with thiol moieties in an analyte solution resulting in an unintended mass shift in the mass spectrum, and oxidation of a sacrificial metal electrode introduces metal ions in the solution that may act as complexing agents thereby changing the characteristics of the mass spectrum.